Occasionally, a disturbance occurs on a utility system, or other power network, that results in a significant voltage drop for a short duration (typically less than 500 ms). Such a disturbance is generally caused by a fault some place in the transmission or distribution system. Faults can be caused, for example, by a single phase conductor being inadvertently connected to ground or the inadvertent connection or short circuiting of multiple phase conductors. These types of faults commonly occur due to equipment failure, bad weather, a vehicular accident, etc. A significant reduction in voltage, sometimes referred to as a sag, can also occur when a large electrical load is energized, such a large motor, or when a large power plant is suddenly disconnected. Smaller faults, sometimes referred to as “dips,” may also occur as a result of other events such as, for example, the switching of capacitors. In any event, whether the fault is large or small, the fault precipitates low-voltage or zero-voltage conditions at various points on the power network.
Managing how a power source supplying power to the power network reacts to low-voltage or zero-voltage events is an important consideration for power source operators. FIG. 1 is an exemplary plot 10 of the voltage level over time across the AC terminals of a power source, such as a wind power unit, connected to a utility grid for an exemplary voltage disturbance caused by a fault. In this example, the fault occurs at approximately time 14, here at t=0.00 seconds, at some electrical distance from the power source, and with the voltage beginning to recover at time 18, here at t˜0.30 seconds. In general, and as shown in FIG. 1, faults cause a generally square-shaped dip or sag 20 in the voltage level between the pre-fault voltage level 24 and the voltage level during recovery 28. It is noted that since a utility grid is a complex impedance network of transmission lines and generators, the actual voltage after the fault tends to overshoot and ring around the utility grid's operating voltage, as illustrated by recovery 28. Those skilled in the art will appreciate that the depth of the voltage dip or sag 20 is generally related to the distance, electrically speaking, between power source and the fault locations. Closer faults cause deeper dips and sags.
For smaller power sources, such as individual wind power units and small wind farms, domestic solar systems, diesel generators, etc., it has been acceptable and desirable (for the owners of the smaller power sources) for the power source to go offline when a voltage reduction of a certain magnitude and of a certain duration occurs. Generally, this operational construct has been acceptable because the total amount of power being provided by the smaller power sources has been relatively small in comparison with the total amount of power provided by other power sources on the power network, such as coal burning power plants, nuclear power plants, etc. Because of this relatively small power producing capability, going offline had little, if any, impact on the recovery ability of the power network after a fault occurred.
As the amount of power coming from these smaller power sources on power networks has been increasing, maintaining their input during, and especially after, a fault or surge has become increasingly important because the repercussions associated with a fault can be exacerbated by a significant amount of power generating capacity going offline in response to the fault. Problems such as frequency swings or large system-wide instabilities of power-generating systems can lead to the disruption of power to large regions, affecting large numbers of power customers. Thus, utility operators (and regulators, see, e.g., Federal Energy Regulatory Commission (FERC) Order 661-A (issued Dec. 12, 2005)) are beginning to require that power sources on their power networks remain online and “ride through” low-voltage and zero-voltage conditions—requirements traditionally applied to common utility power sources, such as fossil-fueled power plants.